Study on Bearing Capacity of Tank Foundation with Alternatively Arranged Vortex-Compression Nodular Piles

energies

Article Study on Bearing Capacity of Tank Foundation with Alternatively Arranged Vortex-Compression Nodular Piles

Chun-Bao Li *, Gao-Jie Li, Ran-Gang Yu, Jing Li and Xiao-Song Ma

Department of Civil Engineering, China University of Petroleum (East China), Qingdao 266580, China; [email protected] (G.-J.L.); [email protected] (R.-G.Y.); [email protected] (J.L.); [email protected] (X.-S.M.) * Correspondence: [email protected]; Tel.: +86-532-8698-1820

Received: 8 September 2020; Accepted: 8 October 2020; Published: 11 October 2020

Abstract: Types of tapered piles are widely applied in tank foundation consolidation, but their inherent deficiencies in design and construction limit their further promotion. Vortex compression pile is a novel nodular pile. Compared with the traditional equal-section pile, vortex compression nodular pile is featured by stronger bearing capacity and slighter settlement. In this paper, the model test results showed that vortex compression nodular pile can greatly improve the bearing capacity and reduce the settlement. Through the finite element software ABAQUS analysis the bearing characteristics of equal-section pile foundation and vortex-compression nodular pile foundation were compared. The three-dimensional solid model was established by ABAQUS finite element software. The impact of cushion modulus, cushion thickness, vertical load, pile modulus, soil modulus around the pile on the bearing capacity of the vortex-compression nodular pile foundation were studied.

Keywords: vortex-compression nodular pile; composite foundation; bearing capacity; tank foundation

1. Introduction In China, most storage tanks in oil and gas reserve projects, with a volume larger than 105 m3, are built on soft foundation area [1]. These tanks are featured by large volume and heavy load, putting strict requirements on the bearing capacity of tank foundation and the control of uneven settlement [2]. At present, because of improper site selection, poor foundation consolidation, and improper foundation structure form, uneven settlement of tank foundation has become outstanding, which results in tank body tilt, floating roof displacement and sticking, tank body distortion, and other related problems [3,4]. Therefore, how to strengthen the foundation, improve the bearing capacity, and mitigate the uneven settlement while saving resources have been the focuses of civil engineering workers [5]. Through the analysis of compaction test in specific projects, Lin P. concluded that gravel pile reinforcement can improve the bearing capacity of tank foundation by compacting soil, which is simple in construction, mature in technology, and can save a lot of cement and steel [6]. Jia J carried out static load test, dynamic penetration test, and standard penetration test on gravel pile composite foundation. They concluded that the modulus of cushion under flexible foundation should fall in the range of 80–200 MPa and reported critical pile length for certain load capacity and strain. They also suggested that appropriate replacement rate can give full play to the load capacity of soil [7]. Combined with the specific engineering geology and trial pile result data of the Cement Fly-ash Gravel(CFG) pile composite foundation of Dongying storage tank, Dong H. obtained the settlement of the foundation under circular uniform load and the load transfer law of the pile body [8]. Song X. simulated the CFG pile by ADINA finite element software. He found that the force mode of side pile and middle pile are not exactly the

Energies 2020, 13, 5273; doi:10.3390/en13205273 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 18 of the foundation under circular uniform load and the load transfer law of the pile body [8]. Song X. simulated the CFG pile by ADINA finite element software. He found that the force mode of side pile and middle pile are not exactly the same, so their load transfer law is also different. Besides, the pile- soil stress ratio is directly proportional to the cushion modulus and increasing the cushion thickness Energies 2020, 13, x FOR PEER REVIEW 2 of 18 or reducing the cushion modulus can mitigate the stress concentration [9]. Li P. simulated the Energies 2020 13 vibroflotationof the foundation, g,ravel 5273 under pile circular by PLAXIS uniform finite load elementand the load software transfer, tlawhe of differential the pile body settlement [8]. Song2 of X 18under. differentsimulated pile length,the CFG replacem pile by ADINAent rate, finite pile element diameter software and pile. He foundspacing that are the studied. force mode He concludedof side pile that the differentialsame,and middle so their pilesettlemen load are transfer nott exactlyis lawnegatively is the also same di ffcorrelatederent., so the Besides,ir load with transfer the the pile-soil replacement law stressis also ratio different rate is,directly and. Besides, pile proportional diameter the pile- [10]. Throughtosoil the stress cushionnumerical ratio modulusis directly simulation, and proportional increasing Liu H. to the the concluded cushion cushion thickness modulus that the or and reducing influence increasing the of cushionthe tank cushion modulus group thickness effect can on foundationmitigateor reducing settlement the stress the cushion concentration is negatively modulus [9 correlated ]. can Li P.mitigate simulated with the the the stress load vibroflotation concentration on the tank gravel foundation[9]. pile Li byP. simulated PLAXIS [11]. finite the elementvInibroflotation 2014, software, Li C. getravel theal. proposed dipffileerential by P theLAXIS settlement vortex finite undercompression element different software nodular pile length,, the pile, replacement differential developed rate,settlement the pile equipment diameter under and toolsanddifferent for pile construction, spacingpile length, are secured studied.replacem Heentnational concludedrate, pile invention diameter that the patent, and diff erentialpile a spacingnd settlementput areforward studied. is negatively a Henew concluded method correlated thatfor tank foundationwiththe differential the consolidation, replacement settlemen rate, tthat is and negatively is, pile the diameter vortex correlated c [ompression10]. Throughwith the nodularreplacement numerical pile simulation, rate foundation, and pile Liu diameterwith H. concluded alternatively [10]. Through numerical simulation, Liu H. concluded that the influence of tank group effect on arrangedthat the nodular influence parts of tank [12– group14]. Through effect on foundationindoor model settlement tests, the is negatively load transfer correlated mechanism with the of load vortex- foundation settlement is negatively correlated with the load on the tank foundation [11]. compressionon the tank nodular foundation pile [and11]. the distribution law of side friction along the pile under different loads InIn 2014, 2014, Li Li C. C. et et al. al. proposed proposed the the vortex vortex compression compression nodular nodular pile, pile, developed developed the theequipment equipment and are studied. This paper focuses on the vertical bearing capacity and the settlement behavior of vortex- andtools toolsfor construction, for construction, secured secured national national invention invention patent,patent, and put and forward put forward a new method a new methodfor tank compression nodular pile as well as the development law of the plastic failure zone of the soil around forfoundation tank foundation consolidation, consolidation, that is, the that vortex is, thecompression vortex compression nodular pile nodular foundation pile with foundation alternatively with the nodularalternativelyarranged partnodular arrangeds [15 parts–18] nodular [12. Through–14]. parts Through [ the12– 14 numerical indoor]. Through model simulation indoor tests, model the load of tests, ABAQUS transfer the load mechanism software, transfer mechanism of the vortex influence- factorsofcompression vortex-compressionof pile foundation nodular pile nodular with and thealternatively pile distribution and the arranged distributionlaw of side nodular friction law of along sideparts frictionthe are pile analyzed. alongunderthe different The pile foundat under loads ion settlementdiareff erentstudied. and loads Thpileis are -papersoil studied. stress focus Thisesratio on paper theunder vertical focuses different bearing on thepile capacity vertical modulus, and bearing the soil settlement capacitymodulus andbehavior around the settlement of pile, vortex cushion- modulus,behaviorcompression cushion of vortex-compression nodular thickness pile asand well load nodularas the are development studied pile as well[19 –law as21] theof. the development plastic failure law zone of the of the plastic soil around failure zonethe nodular of the soilpart arounds [15–18] the. Through nodular partsthe numerical [15–18]. Through simulation the of numerical ABAQUS simulation software, the of ABAQUS influence 2. Experimentalsoftware,factors of the pile influence Work foundation factors with of pilealternatively foundation arranged with alternatively nodular parts arranged are nodularanalyzed. parts The are foundat analyzed.ion Thesettlement foundation and pile settlement-soil stress and ratio pile-soil under stress different ratio under pile modulus, different pilesoil modulus,modulus soilaround modulus pile, cushion around 2.1. Tpile,modulus,echnical cushion Informationcushion modulus, thickness of cushion Pile and Forming thickness load are andstudied load [19 are–21] studied. [19–21].

2. Experimental Work 2.1.1. Pile Forming Mechanism 2.1.Figures T Technicalechnical 1 and Information 2 present ofof PilethePile FormingpileForming forming mold and structure diagram of the nodular pile.

2.1.1. Pile Forming Mechanism Figures1 1 and and2 2present present the the pile pile forming forming mold mold and and structure structure diagram diagram of of the the nodular nodular pile. pile.

(a) (b)

Figure 1. Pile forming mold and structure(a) diagram of the nodular(b) pile: (a) side view; (b) overhead view. Figure 1.1. PilePile formingforming mold mold and and structure structure diagram diagram of theof the nodular nodular pile: pile (a): side (a) view;side view; (b) overhead (b) overhead view. view.

FigureFigure 2. 2.WorkingWorking principle principle diagramdiagram of of pile pile forming forming mold. mold. Figure 2. Working principle diagram of pile forming mold.

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With the help of the rotating S-shaped blade as shown in Figure 2, the pile-forming tool of the nodularWith pile the creates help of a the vortex rotating and squeezes S-shaped the blade concre as shownte in the in pipe Figure into2, the the surrounding pile-forming soil tool so of as the to nodularform the pile nodular creates part. a vortex and squeezes the concrete in the pipe into the surrounding soil so as to form the nodular part. 2.1.2. Pile-Forming Process 2.1.2. Pile-Forming Process The pile-forming equipment of vortex-compression nodular pile mainly includes two parts: The pile-forming equipment of vortex-compression nodular pile mainly includes two parts: vortex-compression steel pipe and pile frame; the vortex-compression steel pipe is composed of knob vortex-compression steel pipe and pile frame; the vortex-compression steel pipe is composed of knob gear, steel pipe sleeve, vortex-compression blade, vortex-compression chamber, and precast concrete gear,pile steelhead; pipe the sleeve,pile frame vortex-compression is composed of pile blade, hammer, vortex-compression steel sleeve pipe chamber, cap, steel and casing, precast column, concrete pilediagonal head; brace, the pile and frame engineering is composed car, etc., of as pile shown hammer, in Figure steel 3. sleeve pipe cap, steel casing, column, diagonal brace, and engineering car, etc., as shown in Figure3.

(a) (b)

(e) (f)

FigureFigure 3. 3.Pile Pile forming forming equipment equipment of the of nodularthe nodular pile: (apile:) schematic (a) schematic diagram diagram of pile frame; of pile (b )frame; schematic (b) diagramschematic of vortex-compressiondiagram of vortex-compression steel pipe; (c) steel physical pipe; drawing (c) physical of vortex-compression drawing of vortex-compression chamber; (d) 3D diagramchamber; of ( vortex-compressiond) 3D diagram of vortex-compression blade; (e) 3D drawing blade; of precast (e) 3D concretedrawing pile of precast head; ( fconcrete) plan of pile knob head; gear. (f) plan of knob gear.

Energies 2020, 13, x 5273 FOR PEER REVIEW 44 of 18 Energies 2020, 13, x FOR PEER REVIEW 4 of 18 The construction process is shown in Figure 4. The construction process is shown in Figure4. The construction process is shown in Figure 4.

Figure 4. The process of construction technology of nodular piles. FigureFigure 4.4. TheThe processprocess ofof constructionconstruction technologytechnology ofof nodularnodular piles.piles. Step 1: Determine the position using the concrete precast pile head and make the pile steel pipe in StepStep 1: place. Determine1: Determine the the position position using using the concretethe concrete precast precast pile headpile head and makeand make the pile the steel pile pipe steel in pipe place. in StepStep 2: 2:Driveplace. Drive the the steel steel casing casing to to the the designated designated depth depth by by using using the the pile pile frame frame equipment, equipment, and and remove remove the Stepthe hammering2: Drive hammering the equipment.steel equipment. casing to the designated depth by using the pile frame equipment, and remove StepStep 3: 3:Injectthe Inject hammering pre-calculated pre-calculated equipment. volume volume of of concrete concrete into into the the steel steel pipe pipe through through the the pipeline. pipeline. StepStepStep 4: 4: Install3: Install Inject the prethe rotary -rotarycalculated torsion torsion volume equipment equipment of concrete and and rotate rotate into the the the steel steel steel pipe pipe pipe to through to form form the thethe first pipeline.first nodular nodular part. part. StepStepStep 5: 5: Slowly4: Slowly Install rotate rotatethe rotary the the steel steel torsion pipe pipe equipment in in the the same same and direction direction rotate and theand steel liftlift itit pipe toto the to next form depth the first whe where nodularre the nodular nodular part. Stepis is5: designed. designed.Slowly rotate the steel pipe in the same direction and lift it to the next depth where the nodular StepStep 6: 6:Continueis Continuedesigned. to to rotate rotate the the steel steel pipe pipe to to form form the the nextnext nodular nodular part part by by using using the rotating and Steptwisting twisting 6: Continue equipment. equipment. to rotate the steel pipe to form the next nodular part by using the rotating and StepStep 7: 7:Injecttwisting Inject the the equipment. concrete concrete until until it it returns returns to to the the pilepile toptop elevation,elevation, rotaterotate and retrieve the the steel steel pipe, pipe, Stepand and7: Inject finally finally the place place concrete the the reinforcement reinforcementuntil it returns cage. cage. to the pile top elevation, rotate and retrieve the steel pipe, and finally place the reinforcement cage. The vortex-compression nodular pile is thus formed. The vortex-compression nodular pile is thus formed. The vortex-compression nodular pile is thus formed. 2.2.2.2. Experimental Experimental Work Work Set Set Up Up 2.2. Experimental Work Set Up TheThe iron iron model model box box with with a a wall thickness of 10 mm, inner diameter of 1000 mm, and height of The iron model box with a wall thickness of 10 mm, inner diameter of 1000 mm, and height of 1200 mm ( (FigureFigure5 5aa)) waswas placedplaced onon thethe rigidrigid floorfloor inin thethe laboratory.laboratory. FourFour holesholes werewereevenly evenly drilleddrilled in in 1200 mm (Figure 5a) was placed on the rigid floor in the laboratory. Four holes were evenly drilled in thethe bottom bottom plate plate of of the box box.. P Porousorous stones placed under the bottom plate allowed the drainage of thethe the bottom plate of the box. Porous stones placed under the bottom plate allowed the drainage of the supersaturatedsupersaturated water water in in the foundation soil (Figure(Figure5 5bb).). LongitudinalLongitudinal andand transversetransverse stistiffeningffening ribsribs supersaturated water in the foundation soil (Figure 5b). Longitudinal and transverse stiffening ribs were fittedfitted onon thethe outer outer wall wall to to enhance enhance the the rigidity rigidity of theof the model model box. box. The The box box was was bigenough big enough to hold to were fitted on the outer wall to enhance the rigidity of the model box. The box was big enough to holdsoil and soil piles.and piles. hold soil and piles.

(a) (b) (a) (b) Figure 5. MModelodel box box:: ( (aa)) physical physical drawing drawing of of model model box box;; and and ( (bb)) model model box box drain drainageage system system.. Figure 5. Model box: (a) physical drawing of model box; and (b) model box drainage system. A load cell (of 100 kN capacity) was fitted on the top of the test pile. Above the load cell lied a plunger A load cell (of 100 kN capacity) was fitted on the top of the test pile. Above the load cell lied a plunger

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Energies 2020, 13, x FOR PEER REVIEW 5 of 18 Energies 2020, 13, x FOR PEER REVIEW 5 of 18 A load cell (of 100 kN capacity) was fitted on the top of the test pile. Above the load cell lied connected to the steel frame. The plunger applied a downward reactive force to the pile. This force was aconnected plunger connectedto the steel to frame. the steel The frame. plunger The applied plunger a downward applied a downwardreactive force reactive to the forcepile. toTh theis force pile. was regulated by a hydraulic jack which was attached to the top of the steel frame. A load sensor with dial Thisregulated force wasby a regulatedhydraulic by jack a hydraulicwhich was jack attached which wasto the attached top of the to the steel top frame. of the A steel load frame. sensor A with load dial indicators was fitted at the top of the pile. Two dial indicators were fixed on a special dial indicator sensorindicator withs was dial fitted indicators at the was top fitted of the at thepile top. Two of thedial pile. indicator Two dials were indicators fixed on were a specialfixed on dial a special indicator bracket through a magnetic stand (Figure 6). The dial indicator bracket was placed on the flat and rigid dialbracket indicator through bracket a magnetic through stand a magnetic (Figure stand6). The (Figure dial indicator6). The dial bracket indicator was placed bracket on was the placed flat and on rigid ground. theground. flat and rigid ground.

Figure 6. Dial indicator layout scheme. Figure 6.6. DialDial indicator layout scheme. 2.3. Equal-Section Pile and Nodular Pile 2.3. Equal Equal-Section-Section Pile and Nodular Pile Based on the similarity of elastic modulus of the materials used under laboratory and real Based on the similarity similarity of elastic elastic modulus modulus of the the materials materials used under laboratory and real conditions and comprehensively considering the experimental material’s ease of processing, low conditions and comprehensively considering the experimentalexperimental material’s ease of processing, low economic cost, and stability, aluminum pipe is selected as the material of model pile. The size of economic cost,cost, and and stability, stability, aluminum aluminum pipe pipe is selectedis selected as the as materialthe material of model of model pile. Thepile size. The of size model of model pile is 30 mm in diameter, 3 mm in wall thickness, 1000 mm in length, and 900 mm in effective pilemodel is 30pile mm is 30 in diameter,mm in diameter, 3 mm in 3 wallmm in thickness, wall thickness, 1000 mm 1000 in length,mm in length, and 900 and mm 900 in mm effective in effective length. length. The material of the expanding part is nylon resin. The height and diameter of the expanding Thelength. material The mater of theial expanding of the expanding part is nylon part is resin. nylon The resin. height The and height diameter and diameter of the expanding of the expanding part are part are 60 mm and 120 mm respectively. In order to increase the friction coefficient between pile and 60part mm are and 60 mm 120 and mm 120 respectively. mm respectively. In order In toorder increase to increase the friction the friction coeffi coefficientcient between between pile andpile soil,and soil, the surface of aluminum pile was coarsened by a small hacksaw. A reserved hole with a radius thesoil, surface the surface of aluminum of aluminum pile pi wasle was coarsened coarsened by a by small a small hacksaw. hacksaw. A reserved A reserved hole hole with with a radius a radius of of 5 mm is reserved near the pile body, which is used as the passage of foil gauge wire. Figure 7 5of mm 5 mm is reserved is reserved near near the pilethe body,pile body, which which is used is asused the passageas the passage of foil gauge of foil wire. gauge Figure wire.7 presentsFigure 7 presents the detailed configuration of pile. thepresents detailed the configurationdetailed configuration of pile. of pile.

(a) (b) (a) (b) Figure 7. TypicalTypical pile structure structure:: ( a) P P-1-1 equal equal-section-section pile; and ( b) P-7P-7 nodular pile. Figure 7. Typical pile structure: (a) P-1 equal-section pile; and (b) P-7 nodular pile. 2.4. Sand 2.4. Sand River sand passingpassing thethe 1 1 mm mm and and 0.1 0.1 mm mm standard standard sieves sieves were were taken taken as theas the foundation foundation soil soil for thisfor River sand passing the 1 mm and 0.1 mm standard sieves were taken as the foundation3 soil for3 thistest. test. The maximumThe maximum and minimumand minimum dry bulk dry densitiesbulk densities of the of sand the were sand 16.30 were kN 16.30/m kN/mand 13.703 and kN 13.70/m this test. The maximum and minimum dry bulk densities of the sand were 16.30 kN/m3 and 13.70 kN/m3 respectively, and the corresponding values of minimum void ratio and maximum void ratio kN/m3 respectively, and the corresponding values of minimum void ratio and maximum void ratio were 0.35 and 0.28 respectively. The moisture content was 21.5%, the specific gravity was 2.66, the were 0.35 and 0.28 respectively. The moisture content was 21.5%, the specific gravity was 2.66, the

Energies 2020, 13, 5273 6 of 18 respectively, and the corresponding values of minimum void ratio and maximum void ratio were 0.35 and 0.28 respectively. The moisture content was 21.5%, the specific gravity was 2.66, the average particle size was 0.21 mm, the effective particle size was 0.11 mm, the nonuniform coefficient was 2.35, the curvature coefficient was 0.82, the cohesion was 4 kPa, and the internal friction angle was 30◦ [22].

2.5. Test Procedure Prior to soil filling, lines with equal spacing were marked on the inner wall of the model box to ensure that soil was filled and impacted layer by layer. The relative compactness of each soil layer was 0.67. The pile was embedded into the soil at designed depth during soil filling. The portal-rigid frame with two columns was used to apply the reaction force to the pile. The column footings were anchored to rigid ground via anchor rod to ensure the stability of the steel frame. Holes drilled on the steel column from top to bottom allowed to adjust the height of the loading system to meet the actual loading requirements. The vertical displacement of the pile was measured by a numerical control displacement sensor with an accuracy of 0.01 mm. The fixed bracket was placed at the edge of the model box, and the magnetic base was attached to the extension arm of the fixed support. Finally, the displacement sensor was installed (Figure7). Loading scheme of model test was as follows: according to technical code for testing of building pile foundation, the load was applied by manual pressure pump stage by stage. As the requirements, the next stage load can only be applied unless the settlement becomes stable. The readings were recorded once every 15 min within the first hour and every 30 min within the second hour, and every 1 h after 2 h. If the settlement at a certain loading stage is less than 0.1 mm within 30 min, the settlement can be regarded as stable and the next loading stage can be applied. Criteria for ultimate load: (1) If at the ith loading stage, the total settlement ∆hi 40 mm, or the ≥ ∆hi is greater than or equal to 5 times of ∆hi 1, the test can be terminated and the load at the (i 1)th − − stage is taken as the ultimate load; (2) if at the ith loading stage, the total settlement ∆hi < 50 mm but the settlement fails to reach a stable state after 24 h, the load test can also be terminated and the load at the (i 1)th stage is taken as the ultimate load. − Parameters of piles are listed in Table1. P1 is the equal-section pile and others are expanded piles. P2–P4 have single nodular part and P5–P7 have two nodular parts. In this paper, the differences in axial force curve and load settlement curve of equal-section pile and nodular piles were compared. Those curves of nodular piles with single and two nodular parts were also compared. In addition, the influence of the position of the nodular part on the ultimate bearing capacity and axial force of the pile body were studied.

Table 1. The conﬁguration of nodular piles.

Eﬀective Pile Diameter of Number of Bottom Elevation Distance Number Pile Length Diameter Nodular Nodular of Nodular Part Between Nodular (mm) (mm) Part (mm) Part (mm) Parts (mm) P1 900 30 - - - - P2 900 30 120 1 810 - − P3 900 30 120 1 570 - − P4 900 30 120 1 330 - − P5 900 30 120 2 330, 630 240 − − P6 900 30 120 2 510, 810 240 − − P7 900 30 120 2 390, 810 360 − −

3. Results and Discussions

3.1. Load Displacement Curve The load settlement curves of all piles are shown in Figure8. Energies 2020, 13, 5273 7 of 18

As can be seen from Figure8, the ultimate bearing capacity of P1 pile is the lowest, while the ultimate bearing capacity of P2 nodular pile reaches 14.4 kN, which is three times that of P1 pile. For P7 with two nodular parts, it has approximately seven times bearing capacity of P1 pile. This illustrates that the nodular piles have much greater bearing capacity than equal-section pile. Besides, the ultimate bearing capacity of piles with two nodular parts are obviously greater than those of piles with single nodular part, indicating that increasing number of nodular parts helps increase the ultimate bearing capacity of nodular piles. It is noticeable that the ultimate load of P7 pile is about 30% higher than that of P5 and P6 piles, which indicates that the distance between the two nodular parts is the key factor Energiesaﬀ ecting2020, 13 the, x FOR bearing PEER capacity REVIEW of piles with double nodular parts [23]. 7 of 18

FigureFigure 8. Load 8. Load-displacement-displacement curve curve of seven kindskinds of of pile pile under under vertical vertical load. load.

Under the same load, the settlement of nodular pile is obviously lower than that of the equal-section As can be seen from Figure 8, the ultimate bearing capacity of P1 pile is the lowest, while the pile. The trend of load settlement curve of P2–P4 piles are similar, which indicates that the position of ultimate bearing capacity of P2 nodular pile reaches 14.4 kN, which is three times that of P1 pile. For nodular part has little inﬂuence on the ultimate load of single nodular piles. This is also veriﬁed by P5 P7 withand P6 two piles, nodular which have parts, almost it has the approximatelysame settlement under seven same times vertical bearing load. capacity For P7 pile of with P1 larger pile. This illustratesdistance that between the nodular nodular piles parts have than P5much and greater P6, a smaller bearing settlement capacity is observed. than equal When-section the distance pile. Besides, is the ultimatelarge, the bearing nodular capacity parts allows of piles the soil with between two nodular them to giveparts full are play obviously to the shear greater capacity. than Whenthose theof piles withdistance single isnodular small to part, a certain indicating range, shear that failure increasing tends to number occur in of the nodular soil body parts between helps the increase nodular the ultimateparts, bearing and thus capacity the shear of capacity nodular of thepiles. soil It body is noticeable might be severelythat the damaged. ultimate load of P7 pile is about 30% higher than that of P5 and P6 piles, which indicates that the distance between the two nodular 3.2. Axial Force Analysis of Monopile parts is the key factor affecting the bearing capacity of piles with double nodular parts [23]. UnderThe the distribution same load, of axialthe settlement force along of the nodular pile body pile of sevenis obviously types of lower piles is than plotted that in of Figure the 9equal. - sectionThe pile. data collectionThe trend of of axial load force settlement along the curve pile body of P2 is– summarizedP4 piles are in similar, Table2. which indicates that the position of nodular part has little influence on the ultimate load of single nodular piles. This is also verified by P5 and P6 piles, which have almost the same settlement under same vertical load. For P7 pile with larger distance between nodular parts than P5 and P6, a smaller settlement is observed. When the distance is large, the nodular parts allows the soil between them to give full play to the shear capacity. When the distance is small to a certain range, shear failure tends to occur in the soil body between the nodular parts, and thus the shear capacity of the soil body might be severely damaged.

3.2. Axial Force Analysis of Monopile The distribution of axial force along the pile body of seven types of piles is plotted in Figure 9. The data collection of axial force along the pile body is summarized in Table 2.

Figure 9. Axial force transfer curve of seven kinds of pile under the ultimate vertical load.

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Table 2. The distribution of axial force along the pile body.

Point 1 Depth (mm), Number Corresponding Axial Point 2 Point 3 Point 4 Point 5 Point 6 Point 7 Force (kN) P1 0, 5.4 200, 5.3 360, 5.3 600, 5.3 760, 5.3 820, 5.3 900, 2.7 − − − − − − P2 0, 15.0 100, 14.9 420, 14.9 650, 14.7 790, 14.2 900, 2.8 950, 2.7 − − − − − − P3 0, 14.4 100, 14.3 560, 14.3 680, 4.9 830, 3.0 890, 2.6 970, 1.6 − − − − − − P4 0, 12.8 100, 12.8 290, 12.8 530, 3.6 700, 2.9 880, 2.7 980, 1.7 − − − − − − P5 0, 22.7 280, 22.6 340, 18.7 520, 17.6 620, 17.2 740, 4.2 980, 1.6 − − − − − − P6 0, 22.7 270, 22.7 470, 22.7 590, 16.7 790, 15.6 910, 2.8 980, 2.2 − − − − − − P7 0, 29.1 370, 29.2 470, 24.1 630, 22.9 790, 20.0 910, 3.4 980, 2.9 − − − − − −

It can be seen from Figure9 that the axial force of equal-section pile P1 keeps almost constant along the pile body until it drops sharply near the pile end. The friction coefficient between the aluminum pile and sandy soil is very small, thus the side friction force along the pile is very small. For this reason, the axial force transfer curves of the equal section pile and the axial force curve of nodular piles above the nodular parts are nearly straight lines. However, for piles with single nodular part (P2–P4), one inflection point can be found on the axial force curves and for piles with two nodular parts (P5–P7), two corresponding inflection points can be observed. It can be seen that the depth of inflection point is well related to the depth of nodular parts, which indicates that the axial force suddenly changes at the nodular part. The reason for this phenomenon is that the nodular part can transfer most of the load to the soil under the nodular part of the pile, which can give full play to the bearing capacity of the soil under the nodular part. From the axial force transfer curve of P2–P4 piles, it can be seen that during the loading process, the nodular part squeezes the soil layer below and in return, the pile subjects to greater lateral force from the soil under the nodular part. As a result, the side friction resistance increases and the axial force reduces at the depth below nodular part. Nodular pile P5 and P6 have approximate difference of axial force at the upper and lower section of nodular part because they have the same distance, i.e., the same soil thickness between nodular parts. The thicker soil layer between the nodular parts of P7 pile holds more load, thus the difference of axial force at the upper and lower section of nodular part is much greater. Axial force reduction below the depth of first nodular part of pile with double nodular parts is more significant than that of pile with single nodular part. When the soil between the two nodular parts is squeezed by the vertical load, the lower nodular part hinders the downward movement of the soil, resulting in greater pressure on this part of the soil. Therefore, the lateral reaction force at this section is larger than that of the single nodular part pile, and the side friction resistance of this part is relatively larger. Therefore, the reduction of axial force at this part of pile with double nodulars is larger than that of the single nodular part pile.

4. Simulations In order to test that the foundation consolidated by the vortex-compression nodular pile can improve the bearing capacity and reduce the settlement, the influence of different loads, pile modulus, modulus of soil around pile, cushion modulus, and cushion thickness on the pile-soil stress ratio and settlement of the complexes foundation were studied and analyzed. The finite element analysis was carried out by ABAQUS software [24–26].

4.1. Finite Element Model Description P5 pile and P6 pile were used for ﬁnite element simulation. Their geometric scheme is shown in Figure 10. The P5 and P6 were embedded alternatively, as shown in Figure 11. The selected numerical simulation unit is shown in Figure 12. According to Figure 12, the numerical simulation model is an axisymmetric model. In order to improve computation eﬃciency, a quarter of the model is selected for analysis. As the assumptions, the soil around the pile and the cushion conform the ideal elastic-plastic Energies 2020, 13, x FOR PEER REVIEW 9 of 18 Energies 2020, 13, x FOR PEER REVIEW 9 of 18 4.1.Energies Finite 2020 Element, 13, x FOR Model PEER Description REVIEW 9 of 18 4.1. Finite Element Model Description 4.1. FiniteP5 pile Element and P6 Model pile were Description used for finite element simulation. Their geometric scheme is shown in FigureP5 10. pile The and P5 andP6 pile P6 were were embedded used for finite alternatively, element simulation. as shown in Their Figure geometric 11. The selected scheme numericalis shown in P5 pile and P6 pile were used for finite element simulation. Their geometric scheme is shown in simulationFigure 10. unitThe P5is shown and P6 in were Figure embedded 12. According alternatively, to Figure as shown12, the innumerical Figure 11. simulation The selected model numerical is an Figure 10. The P5 and P6 were embedded alternatively, as shown in Figure 11. The selected numerical axisymmetricsimulation unit model. is shown In order in Figure to improve 12. According computation to Figure efficiency, 12, the a numericalquarter of simulationthe model modelis selected is an simulation unit is shown in Figure 12. According to Figure 12, the numerical simulation model is an foraxisymmetric analysis. As model.the assumptions, In order to the improve soil around computation the pile efficiency, and the cushion a quarter conform of the themodel ideal is elasticselected- axisymmetricEnergies 2020, 13model., 5273 In order to improve computation efficiency, a quarter of the model is9 ofselected 18 plasticfor analysis. model Asand the the assumptions, Mohr Coulomb the soilmodel around is adopted. the pile The and pilethe cushionand loading conform plate the are ideal described elastic - for analysis. As the assumptions, the soil around the pile and the cushion conform the ideal elastic- withplastic linear model elastic and model. the Mohr The Coulomb research model scope ofis theadopted. model The is: pile 10 times and loading of pile diameterplate are indescribed radial plastic model and the Mohr Coulomb model is adopted. The pile and loading plate are described direction,withmodel linear twice and elastic the of Mohrpile model. length Coulomb T hein modeldepth, research is and adopted. scope 0.28 m of The in the pilecushion model and loading diameter. is: 10 plate times are of described pile diameter with linear in radial withdirection,elastic linear model. twice elastic Theof model.pile research length The scope in research depth, of the model and scope 0.28 is: of 10m the timesin cushion model of pile is: diameterdiameter. 10 times in radial of pile direction, diameter twice in of radial direction,pile length twice in of depth, pile andlength 0.28 in m depth, in cushion and diameter. 0.28 m in cushion diameter.

Figure 10. Geometric scheme of numerical simulation . Figure 10. Geometric scheme of numerical simulation. FigureFigure 10. 10. GeometricGeometric scheme of of numerical numerical simulation. simulation.

Figure 11. Layout scheme of pile type. Figure 11. Layout scheme of pile type. Figure 11. Layout scheme of pile type. Figure 11. Layout scheme of pile type.

Figure 12. Selected unit for numerical simulation. Figure 12. Selected unit for numerical simulation. (1) Element selection:Figure eight 12. node Selec linearted unit hexahedron for numerical element simulation (C3D8R). is adopted for each Figure 12. Selected unit for numerical simulation. component,(1) Element and selection: the reduced eight integral node is linearselected, hexahedron which can prevent element sand (C3D8R) leakage. isTen adopted node quadric for each component,tetrahedral(1) Element and element the selection: reduced (C3D10) integral eight is used node inis selected, local linear parts. hexahedronwhich The straight can prevent elementsection sand of (C3D8R) the leakage. pile is roughlyis Ten adopted node divided quadric for each (1) Element selection: eight node linear hexahedron element (C3D8R) is adopted for each tetrahedralcomponent,into units. element and The the nodular (C3 reducedD10) part is integral andused the in contactislocal selected, parts. area aroundwhichThe straight can nodular prevent section part aresand of ﬁnelythe leakage. pile divided is roughlyTen into node units. divided quadric component, and the reduced integral is selected, which can prevent sand leakage. Ten node quadric intotetrahedral Finally,units. The the element wholenodular model(C3 partD10) is and divided is usedthe intocontact in local 18031 area parts. units. around The See Figurestraight nodular 13 sectionfor part the are grid of thefinely division pile divided is results. roughly into divided units. tetrahedral element (C3D10) is used in local parts. The straight section of the pile is roughly divided Finally,into units. the wholeThe nodular model partis divided and the into contact 18031 areaunits. around See Figure nodular 13 for part th eare grid finely division divided results. into units. intoFinally, units. the The whole nodular model part is dividedand the intocontact 18031 area units. around See Figurenodular 13 part for tharee gridfinely division divided results. into units. Finally, the whole model is divided into 18031 units. See Figure 13 for the grid division results.

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(a) (b)

FigureFigure 13. 13.Grid Grid division division and and location location sketchsketch map of the model model:: (a (a) )perspective perspective drawing drawing of oflocal local grid grid divisiondivision of of local local soil soil around around pile; pile;( b(b)) perspectiveperspective drawing of of grid grid division division of of expanding expanding part part;; (c) (1/4c) 1 /4 modelmodel parts parts location location diagram; diagram; and and (d(d)) schematicschematic diagramdiagram of of pile, pile, cushion, cushion, and and loading loading plate. plate.

(2)(2) Material Material parameters: parameters: ItIt isis assumedassumed that the soil around around the the pile pile is is homogeneous. homogeneous. The The elastic elastic modulusmodulus of of soil soil around around the the pile pile isis 2020 MPa,MPa, thethe modulus of of load loadinging plate plate is is 40 40 GPa, GPa, the the thickness thickness of of cushioncushion is is 30 30 mm, mm, the the elastic elastic modulus modulus ofof cushioncushion is is 80 80 MPa, MPa, and and the the elastic elastic modulus modulus of ofpile pile is 30 is GPa. 30 GPa. (3)(3) Constraint Constraint conditions: conditions: TheThe soil around the the pile pile is is set set as as a acylindrical cylindrical boundary, boundary, and and the the Y Y directiondirection is is the the vertical vertical direction direction of of the the model.model. X and and Z Z-direction-direction constraints constraints are are set set on on the the side side of soil, of soil, ﬁxedfixed constraints constraints in in X, X, Y, Y, and and Z Z directions directions areare setset on the foundation soil soil surface, surface, the the X- X-directiondirection and and Z-directionZ-direction constraints constraints are are set set on on the the two two sides sides of of the the soil. soil. Before Before balancing balancing the the ground ground stress stress of of the the soil, soil, the X and Z-direction constraints are set at the contact surface between the soil and the pile, and the X and Z-direction constraints are set at the contact surface between the soil and the pile, and the the constraints are deleted after the balance. Before balancing the ground stress of the pile, three constraints are deleted after the balance. Before balancing the ground stress of the pile, three directions directions of constraints are set on the pile, the X and Z-direction constraints are set on the pile after of constraints are set on the pile, the X and Z-direction constraints are set on the pile after the balance. the balance. The cushion and loading plate are deleted in the analysis step of balancing ground stress. The cushion and loading plate are deleted in the analysis step of balancing ground stress. After the After the balance, the cushion and loading plate are restored, and the X and Z-direction constraints balance,are set theon the cushion side of and the loadingcushion plateand loading are restored, plate. and the X and Z-direction constraints are set on the side(4) of Contact the cushion model: and The loading contact plate. between pile and soil conforms to Coulomb friction contact model(4) Contact with the model: friction The coefficient contact betweenof 0.4. Hard pile andcontact soil model conforms is adopted to Coulomb for pile friction top and contact cushion model withcontact, the friction cushion coe andﬃ soilcient around of 0.4. pile Hard contact, contact cushion model and is loading adopted plate for pilecontact. top and cushion contact, cushion(5) and Load soil application: around pile The contact, vertical cushion load is anddirectly loading applied plate on contact. the loading plate in ten stages and the(5) load Load increment application: of each The stage vertical is 80 kPa. load Unloading is directly process applied is on not the considered loading platein this in paper. ten stages and the load(6) increment Ground stress of each balance: stage isBalancing 80 kPa. Unloadingthe initial ground process stress is not is considered a very important in this paper.step in this model.(6) Ground The simulation stress balance: of pile-soil Balancing interaction the must initial be considered. ground stress The is soil a veryweight important in this model step is in set this model.as the Theinitial simulation stress. The of upper pile-soil surface interaction of the model must soil be is considered. plane, so ground The soilstress weight balance in can this be model done is setby as ABAQUS the initial software. stress. The The upper displacement surface of after the balancemodel soil reaches is plane, 10−9 soorders ground of magnitude, stress balance and can the be balance effect is very good.

Energies 2020, 13, x FOR PEER REVIEW 12 of 19 Energies 2020, 13, 5273 11 of 18 as the initial stress. The upper surface of the model soil is plane, so ground stress balance can be done by ABAQUS software. The displacement after balance reaches 10−9 orders of magnitude, and the done by ABAQUS software. The displacement after balance reaches 10 9 orders of magnitude, and the balance effect is very good. − Energiesbalance 2020 eﬀ,ect 13, isx FOR very PEER good. REVIEW 11 of 18 4.2. Comparison of Bearing Capacity of Equal-Section Pile Foundation and Vortex-Compression Nodular Pile 4.2. Comparison of Bearing C Capacityapacity of Equal Equal-Section-Section Pile Foundation and Vortex Vortex-Compression-Compression Nodular Pile Foundation PileFoundation Foundation The modeling process of equal-section pile foundation and vortex-compression nodular pile The modeling process of equal-sectionequal-section pile foundation and vortex-compressionvortex-compression nodular pile foundation are the same. The pile spacing is three times of pile diameter, and the physical parameters foundation are the same. The The pile pile spacing spacing is is three three times times of of pile pile diameter, diameter, and and the the physical physical parameters parameters of soil and pile are the same. The vertical stress and displacement two kinds of foundation are of soil soil and and pile pile are are the the same. same. The The vertical vertical stress stress and displacement and displacement two kinds two of kinds foundation of foundation are extracted are extractedfrom the simulation from the simulationsimulation results, as results,results, shown inasas Figureshownshown 14 inin. FigureFigure 14.14.

Figure 14. The load settlement curve of two complexes foundations. Figure 14. The load settlement curve of two complexe complexess foundation foundations.s.

It can be seen from Figure 14 that the settlement of equal-section pile foundation is larger under It c canan be seen from Figure 14 that the settlement of equal equal-section-section pile foundation is larger under the same load. The curve of complexes foundation with constant cross-section pile is steep, while that the same load. The The curve curve of of complexes complexes foundation foundation with with constant constant cross cross-section-section pile is steep, while that of complexes foundation with enlarged diameter pile is smooth. With the increase of the load, the of complexes foundation foundation with with enlarged enlarged diameter diameter pile pile is issmo smooth.oth. With With the theincrease increase of the of load, the load, the settlement difference between the two complexes foundations becomes larger and larger, which fully settlementthe settlement difference diﬀerence between between the two the twocomplexes complexes foundations foundations becomes becomes larger larger and larger, and larger, which which fully shows that the foundation treated by dislocation vortex compression pile has good settlement showsfully shows that thatthe foundation the foundation treated treated by by dislocation dislocation vortex vortex compression compression pile pile has has good good settlement reduction and greater bearing capacity [27]. reduction and greater bearingbearing capacity [[27].27].

4.3. Simulation Analysis of Influencing Factors on Bearing Characteristics of Vortex-Compression Nodular 4.3. Simulation Analysis of Influencing Inﬂuencing Factors on Bearing Characteristics of Vortex Vortex-Compression-Compression Nodular PilePile FoundationFoundation Pile Foundation

4.3.1. Analysis of InﬂuenceInfluence of Load on Bearing Characteristics 4.3.1. Analysis of Influence of Load on Bearing Characteristics The thickness of of cushion cushion is is 30 30 mm. mm. The The elastic elastic modu moduluslus of ofsoil soil around around the thepile pile is 80 is MPa, 80 MPa, the The thickness of cushion is 30 mm. The elastic modulus of soil around the pile is 80 MPa, the themodulus modulus of loading of loading plate plateis 40 GPa, is 40 the GPa, elastic the elasticmodulus modulus of cushion of cushion is 80 MPa, is 80and MPa, the elastic and the modulus elastic modulus of loading plate is 40 GPa, the elastic modulus of cushion is 80 MPa, and the elastic modulus modulusof pile is 30 of GPa. pile isThe 30 GPa.stress Thenephogram stress nephogram and displacement and displacement nephogram nephogram under 800 underkPa load 800 are kPa shown load of pile is 30 GPa. The stress nephogram and displacement nephogram under 800 kPa load are shown arein Figures shown 15 in Figuresand 16. 15 and 16. in Figures 15 and 16.

Figure 15. Stress cloud of 800 kPa load.

Energies 2020, 13, 5273 12 of 18 EnergiesEnergies 20202020,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 1213 of 1819

FigureFigFigureure 16.1616.. DisplacementDisplacement contour contourcontour of of 800 800800kPa kPakPa load. loadload..

AccordingAccording to toto the thethe results resultsresults of ofof numerical numericalnumerical analysis, analysis,analysis, the thethe ratios ratiosratios of ofof vertical verticalvertical stress stressstress at at topat toptop of of P5of P5P5 pile pilepile and andand at adjacentat adjacentadjacent soil soilsoil under under diﬀ differentdifferenterent loads loadsloads are areare drawn, drawn,drawn, in Figure inin Figure 17. 17.

FigureFigureFigure 17.17.17. TheThe pile-soilpilepile-soil-soil verticalvertical stressstress ratioratioratio underunderunder di differentdifferentﬀerent loads. loadsloads..

ItItIt cancancan bebebe seenseenseen fromfromfrom FigureFigureFigure 17 1717 that thatthat thethethe pile-soil pilepile-soil-soil verticalverticalvertical stressstressstress ratioratioratio increasesincreasesincreases withwithwith thethethe increaseincreaseincrease ofofof load. When the load is small, the soil deformation is in the elastic stage, the pile-soil stress ratio is small. load.load. WhenWhen thethe loadload isis small,small, thethe soilsoil deformationdeformation isis inin thethe elasticelastic stage,stage, thethe pilepile-soil-soil stress ratioratio isis Thesmall.small. bearing TheThe bearingbearing capacity capacitycapacity of the soilofof thethe around soilsoil around the pile thethe works pilepile worksworks well where, wellwell where,where, the cushion thethe cushioncushion plays playsplays an active anan activeactive role. Therole.role. stress TheThe distribution stressstress distributiondistribution is normal isis and normalnormal uniform. andand uniform.uniform. Besides, when Besides,Besides, the load whenwhen is the small,the loadload the isis friction small,small, resistance thethe frictionfriction at theresistanceresistance top of pile atat thethe is negative toptop ofof pilepile under isis negativenegative the action underunder of cushion, thethe actionaction which ofof helps cushion,cushion, to reduce whichwhich the helpshelps deformation toto reducereduce and thethe improvedeformation the bearing andand improveimprove capacity thethe [28 bearingbearing]. When capacitycapacity the load [28].[28]. exceeds WhenWhen 400 thethe kPa, loadload the exceedsexceeds pile-soil 400400 stress kPa,kPa, ratio thethe increases pilepile-soil-soil obviouslystressstress ratioratio with increasesincreases the increase obviouslyobviously of load. withwith At thethe this increaseincrease time, the ofof regulating load.load. AtAt this eff ect time,time, of cushionthe regregulatingulating is obviously effect weakened,of cushion andisis obviouslyobviously the stress weakened,weakened, concentrates andand uponthethe stressstress the concentratesconcentrates top of the pile. uponupon Finally, thethe toptop the ofof thethe plastic pile.pile.failure Finally, of thethe cushion plasticplastic at failurefailure pile topof cushion occurs. atat pilepile toptop occurs.occurs. 4.3.2. Analysis of influence of Cushion Thickness on Bearing Capacity 4.3.2. Analysis ofof influenceinfluence ofof CushionCushion ThicknessThickness onon BearingBearing CapacityCapacity Cushion is the essential part of composite foundation, which can share part of the upper load to Cushion isis thethe essentiaessentiall part ofof compositecomposite foundation,foundation, whichwhich cancan shareshare partpart ofof thethe upperupper loadload toto the soil around the pile and allow the surrounding soil to give full play of the bearing capacity. For this thethe soilsoil aroundaround thethe pilepile andand allowallow thethe surroundingsurrounding soilsoil toto givegive fullfull playplay ofof thethe bearingbearing capacity.capacity. ForFor reason, it can greatly reduce the settlement and avoid the damage caused by the stress concentration of thisthis reason,reason, itit cancan greatlygreatly reducereduce thethe settlementsettlement andand avoidavoid thethe damagedamage causedcaused bbyy the stressstress the pile top on the upper components [29]. The influence of cushion thickness on settlement is shown concentrationconcentration ofof thethe pilepile toptop onon thethe upperupper componentscomponents [29].[29]. TheThe influenceinfluence ofof cushioncushion thicknessthickness onon in Figure 18. settlementsettlement isis shownshown inin FigureFigure 18.18.

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Figure 18. The thickness of the cushion effect on the curve of settlement.

It can be seen from Figure 18 that for a constant cushion thickness, the settlement increases with Figure 18. The thickness of the cushion effect on the curve of settlement. the increase of theFigure load. 18. Under The thickness the constant of the cushion vertical effect load, on thethe curve settlement of settlement decreases. as the cushion thicknessIt increases can be seen from from 20 Figure mm to18 that30 mm. for a At constant this time, cushion the thickness,bearing capacity the settlement of the increases soil between with piles It can be seen from Figure 18 that for a constant cushion thickness, the settlement increases with is fullythe played, increase and of the the load. stress Under is well the constantdistributed. vertical However, load, the further settlement increasing decreases the as cushion the cushion thickness the increase of the load. Under the constant vertical load, the settlement decreases as the cushion will thicknesslead to a increases larger settlement. from 20 mm toIn 30this mm. simulation, At this time, the the optimum bearing capacity cushion of thethickness soil between is about piles is30 mm thickness increases from 20 mm to 30 mm. At this time, the bearing capacity of the soil between piles [30,31].fully played, and the stress is well distributed. However, further increasing the cushion thickness will is fully played, and the stress is well distributed. However, further increasing the cushion thickness leadFigure to a19 larger presents settlement. the influence In this simulation, of cushion the optimumthickness cushion on pile thickness-soil stress is about ratio. 30 The mm trend [30,31 ].of five will lead to a larger settlement. In this simulation, the optimum cushion thickness is about 30 mm curves withFigure different 19 presents thickness the influence is approximately of cushion thicknessthe same. on The pile-soil pile-soil stress stress ratio. ratio The trendincreases of five linearly [30,31].curves with different thickness is approximately the same. The pile-soil stress ratio increases linearly under small loads. When the load reaches 320 kPa, the pile-soil stress ratio increases sharply and even underFigure small 19 presents loads. When the theinfluence load reaches of cushion 320 kPa, thickness the pile-soil on stress pile ratio-soil increasesstress ratio. sharply The and trend even of five increases at an exponential rate after the load exceeds 560 kPa. Under the same load, the thicker the curvesincreases with different at an exponential thickness rate is afterapproximately the load exceeds the same. 560 kPa. The Under pile-soil the samestress load, ratio the increases thicker the linearly cushion is, the smaller the pile-soil stress ratio is and greater the bearing capacity of the soil around the undercushion small is,loads. the smaller When the pile-soilload reaches stress 320 ratio kPa, is and the greater pile-soil the stress bearing ratio capacity increases of the sharply soil around and even pile is. However, considering the influence of cushion thickness on settlement, a thicker cushion is not increasesthe pile at is.an However, exponential considering rate after the the influence load exceeds of cushion 560 thickness kPa. Under on settlement, the same a load, thicker the cushion thicker the alwaysis not favorable always favorablein practical in practicalapplications. applications. In other Inwords, other words,the effect the of eff cushionect of cushion thickness thickness on settlement on cushion is, the smaller the pile-soil stress ratio is and greater the bearing capacity of the soil around the and settlementpile-soil stress and pile-soil ratio should stress ratiobe considered should be considered at the same at the time. same The time. thickness The thickness of cushion of cushion should be pile is. However, considering the influence of cushion thickness on settlement, a thicker cushion is not reasonablyshould be selected reasonably so selected as to maximize so as to maximize the bearing the bearing capacity capacity of soil of soil around around the the pile pile and and adjust adjust the always favorable in practical applications. In other words, the effect of cushion thickness on settlement reasonablethe reasonable stress distribution stress distribution on the on pile the pileand andthe thesoil soil around around the the pile. pile. and pile-soil stress ratio should be considered at the same time. The thickness of cushion should be reasonably selected so as to maximize the bearing capacity of soil around the pile and adjust the reasonable stress distribution on the pile and the soil around the pile.

Figure 19. The thickness of the cushion effect on the pile-soil ratio. Figure 19. The thickness of the cushion effect on the pile-soil ratio. 4.3.3. Influence Analysis of Cushion Modulus on Bearing Capacity 4.3.3. Influence Analysis of Cushion Modulus on Bearing Capacity The thicknessFigure of cushion 19. The is thickness 30 mm. of The the elasticcushion modulus effect on ofthe soil pile around-soil ratio. the pile is 80 MPa, theThe modulus thickness of loadingof cushion plate is is 30 40 mm. GPa, The and theelastic elastic modulus modulus of of soil pile around is 30 GPa. the Thepile influence is 80 MPa, of the 4.3.3.moduluscushion Influence of loading modulus Analysis plate on settlement of is Cushion40 GPa, is and shown Modulus the in elastic Figure on Bearing modulus 20. Capacity of pile is 30 GPa. The influence of cushion modulus on settlement is shown in Figure 20. The thickness of cushion is 30 mm. The elastic modulus of soil around the pile is 80 MPa, the modulus of loading plate is 40 GPa, and the elastic modulus of pile is 30 GPa. The influence of cushion modulus on settlement is shown in Figure 20.

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Figure 20. The elastic modulus of the cushion eﬀect on the curve of settlement.

It can be seen from Figure 20 that the settlement decreases with the increase of cushion modulus under the same load. The reason is that the increase of the cushion modulus is more likely to cause stress concentration on the pile top. The cushion transfers most of the load to the vortex-compression nodular pile, hence, the load shared by the soil is small. The nodular pile plays an excellent role in settlement reduction. The difference between the settlement of pile and the soil around the pile tends to be stable, so the overall settlement is reduced. With the increase of load, the influence of cushion modulus on settlement becomes more obvious. According to Figure 21, the seven curves with different modulus share approximately the same trend. Under the same load, the greater the cushion modulus is, the greater the pile-soil stress ratio is, and the smaller the coefficient of bearing capacity of soil around the pile is. The reason behind this is that the increase of cushion modulus leads to the stress concentration at the pile top. The fluidity of cushion material decreases, thus the gap between pile and soil around pile cannot be filled in time, and the squeezing effect on soil is reduced. In practical application, it is necessary to select a moderate cushion modulus to reduce the settlement and increase the pile-soil stress ratio meanwhile.

Figure 21. The elastic modulus of the cushion eﬀect on the pile-soil ratio.

4.3.4. Inﬂuence Analysis of Modulus of Soil Around Pile on Bearing Characteristics The inﬂuence of elastic modulus of soil around the pile on settlement is shown in Figure 22. Energies 2020, 13, x FOR PEER REVIEW 15 of 18 Energies 2020, 13, 5273 15 of 18

Figure 22. The elastic modulus of the soil effect on the curve of settlement.

It can be seen fromFigure Figure 22. The 22 elastic that modulus under ofthe the same soil eff load,ect on thethe curve settlement of settlement. decreases gradually with the increaseIt of can modulus be seen from of soil Figure around 22 that the under pile. the As same the load, modulus the settlement of soil decreasesaround the gradually pile increases, with the soil is morethe increase difficult of modulusto be compressed of soil around and thethe pile. settlement As the modulusis smaller. of soilIn addition, around the the pile soil increases, at the pile end is not easythe soil to be is morecompressed, difficult to resulting be compressed in the anddecrease the settlement of pile settlement. is smaller. In Under addition, greater the soil load, at the this effect is morepile obvious. end is not easy to be compressed, resulting in the decrease of pile settlement. Under greater load, Asthis show effectn in is moreFigure obvious. 23, when the modulus of soil around the pile is constant, the pile-soil stress As shown in Figure 23, when the modulus of soil around the pile is constant, the pile-soil stress ratio first increases rapidly and then slowly with the increase of load. Under the same load, the greater ratio first increases rapidly and then slowly with the increase of load. Under the same load, the greater the soilthe modulus, soil modulus, the smaller the smaller the pile thepile-soilsoil stress stress ratio ratio is. is.With With the the increase increase of of the the modulus modulus of of soil soil around the pile,around the the compressive pile, the compressive resistance resistance of the of cushion the cushion to to the the soil soil around around the the pile pile increases increases and the and the settlementsettlement of the of soil the decreases, soil decreases, hence hence the the soil soil around around thethe pile pile shares shares more more load. load.

Figure 23. The elastic modulus of the soil effect on the pile-soil ratio. Figure 23. The elastic modulus of the soil effect on the pile-soil ratio. 4.3.5. Influence Analysis of Pile Modulus on Bearing Characteristics 4.3.5. InfluenceThe influence Analysis of pileof Pile modulus Modulus on model on settlementBearing Characteristics and pile-soil stress ratio are shown in Figures 24 Theand influence 25, respectively. of pile modulus on model settlement and pile-soil stress ratio are shown in Figures Under small load, the change of pile modulus has little effect on settlement. Even when the load 24 andreaches 25, respectively. 800 kPa, the settlement decreases by only 2.1 mm. Similarly, it can be seen from Figure 25 that the pile-soil stress ratio slightly increases with the increase of pile modulus. The results show that the pile modulus has little effect on the settlement and pile-soil stress ratio.

Figure 24. The elastic modulus of the pile effect on the curve of settlement.

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Energies 2020, 13, x FOR PEERFigure REVIEW 24. The elastic modulus of the pile eﬀect on the curve of settlement. 16 of 18

Figure 25. The elastic modulus of the pile effect the pile-soil ratio. Figure 25. The elastic modulus of the pile effect the pile-soil ratio. 5. Conclusions UnderThrough small load, the indoor the change model test, of pile the bearingmodulus capacity has little of the effect vortex-compression on settlement. nodular Even pileswhen and the load reachesequal-section 800 kPa, the piles settlement are compared. decreases The curve by ofonly axial 2.1 force mm. distribution Similarly, along it can the be pile seen body from and theFigure 25 that theload-settlement pile-soil stress curve ratio of the slightly pile are increases mainly studied. with Throughthe increase the finite of elementpile modulus. simulation, The the results effect show that theof pile parameters modulus on bearinghas little characteristics effect on the of compositesettlement foundation and pile- withsoil stress vortex-compression ratio. nodular piles are analyzed [32,33]. The following conclusions can be drawn from this paper. 5. ConclusionsThe results show that, compared with equal-section pile foundation, the vortex-compression nodular piles foundation has superior bearing capacity, slighter settlement, and gentler settlement Throughchanges with the theindoor load. Themodel position test, of the nodular bearing part hascapacity little influence of the on vortex the bearing-compression capacity of nodularnodular piles and equalpiles- withsection single piles or doubleare compared. nodular parts. The Nevertheless,curve of axial the force bearing distribution capacity of vortex-compressionalong the pile body and the loadnodular-settlement piles increases curve of with the the pile increase are mainly of the distance studied. between Through the two the nodular finite element parts. simulation, the The settlement is positively correlated with the load but negatively correlated with the modulus effect of parameters on bearing characteristics of composite foundation with vortex-compression of the cushion and the modulus of the soil around the pile. The settlement decreases but then increases nodular piles are analyzed [32,33]. The following conclusions can be drawn from this paper. with the increase of cushion thickness. The pile-soil stress ratio is positively correlated with the load Tandhe results the cushion show modulus that, comparedbut is negatively with correlated equal-section with the pile cushion foundation, thickness, the and vortex soil modulus.-compression nodular pilesIn practical foundation application, has superior the influence bearing of cushion capacity, thickness slighter and cushionsettlement, modulus and ongentler settlement settlement changesand with pile-soil the stressload. ratio The should position be comprehensivelyof nodular part considered, has little andinfluence appropriate on the cushion bearing thickness capacity of nodularand piles modulus with should single be orselected. double nodular parts. Nevertheless, the bearing capacity of vortex- compression nodular piles increases with the increase of the distance between the two nodular parts. The settlement is positively correlated with the load but negatively correlated with the modulus of the cushion and the modulus of the soil around the pile. The settlement decreases but then increases with the increase of cushion thickness. The pile-soil stress ratio is positively correlated with the load and the cushion modulus but is negatively correlated with the cushion thickness, and soil modulus. In practical application, the influence of cushion thickness and cushion modulus on settlement and pile-soil stress ratio should be comprehensively considered, and appropriate cushion thickness and modulus should be selected.

Funding: The research was carried out in the general project “Study on quantitative prediction of fractures in tight reservoirs under the coupling action of thermal, fluid, solid, and creep” (project number 4197-2138) kindly supported by The National Natural Science Foundation of China.

Acknowledgments: We kindly thank Research Center of Civil Engineering, China University of Petroleum (East China) for providing laboratory facilities and experimental devices.

Conflicts of Interest: The authors declare no conflict of interest.

Energies 2020, 13, 5273 17 of 18

Author Contributions: C.-B.L. conceived the study. C.-B.L. and G.-J.L. carried out the case study. R.-G.Y. supervised the study. G.-J.L. and X.-S.M. collected and analyzed the data. C.-B.L. edited the manuscript. J.L. provided some suggestion for the manuscript editing. All authors have read and agreed to the published version of the manuscript. Funding: The research was carried out in the general project “Study on quantitative prediction of fractures in tight reservoirs under the coupling action of thermal, fluid, solid, and creep” (project number 4197-2138) kindly supported by The National Natural Science Foundation of China. Acknowledgments: We kindly thank Research Center of Civil Engineering, China University of Petroleum (East China) for providing laboratory facilities and experimental devices. Conflicts of Interest: The authors declare no conflict of interest.

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